Exhaust gas recirculation systems (referred to hereafter as EGR systems), wherein part of the exhaust gas from an engine is recycled in the air intake to reduce peak temperature and pressure in the combustion chamber, are a recognized means of reducing emission of NOx in the exhaust from direct injection diesel engines.
Although such EGR systems do reduce the amount of NOx generated, they also have the disadvantage that generation of smoke (particulate matter) tends to increase, since the recirculation of exhaust gas effectively reduces the oxygen concentration in the combustion chamber.
The fuel injection valves of diesel engines are generally fitted to the cylinder head at an oblique angle to avoid interference with air intake or exhaust valves. This however leads to uneven fuel injection inside the chamber so that air is used less efficiently where there is a high concentration of fuel, and more smoke is generated when exhaust gas is recycled.
A two stage fuel injection valve for diesel engines is disclosed in, for example, Jikkaisho No. 61-160271 and No. 61-162572 published by the Japanese Patent Office.
In this valve, a small amount of fuel is injected in an initial lift, and after this initial amount of fuel has ignited, a larger amount of fuel is injected in a second lift. This system was intended to reduce the ignition delay in the main fuel injection, help the the combustion flame to spread smoothly and rapidly throughout the chamber, reduce combustion noise and emission of NOx, and generate less smoke.
In this type of fuel injection valve however, if the needle valve is even slightly decentered in the initial injection when the injection pressure is low, the injection of fuel into the chamber is liable to be uneven, conversion of fuel to fine droplets is difficult where fuel is in high concentration, and as air is not used efficiently in the combustion, even more smoke may be generated.
Thus, even if an EGR system is combined with a two stage fuel injection valve and emission of NOx is decreased, smoke generation often increases. It was therefore difficult to reduce both NOx and smoke simultaneously.